Treatment and reduction of concentrations of metals in metal bearing industrial waste streams to environmentally acceptable levels has been a long term problem. It is important to be able to treat such wastes and remove metals, hazardous materials, and toxic substances, with minimal amounts of solid wastes remaining in a cost effective manner. The ultimate solution to such environmental problems, recovery, recycling, and reuse of metals contained within waste streams has been inadequately addressed.
In those instances where metals, compounds, and hazardous materials are not separated from waste streams, but are transported to special waste disposal facilities for treatment or storage, the metals are not recovered, leaving them to be disposed of with other unprocessed or partially processed wastes. As a result, not only is there no recycling with the attendant potential for economic profit or cost reduction, but waste disposal and waste storage problems are created as well. Such waste disposal and waste storage problems are associated with high cost and long waste storage time periods. Often, the wastes generated are considered to be hazardous. Under many environmental statutes, hazardous, toxic, and/or dangerous wastes remain the liability of the waste generator, as long as these wastes exist in the environment. Such long term liability remains with the generator, even though the wastes may have been treated and placed in a secure landfill for disposal.
Processes for removing metals from waste streams including ion exchange and electrolysis have heretofore been known, but theses process are limited. Ion exchange is costly, slow, and cumbersome to use, and in order to be effective, the waste water being treated must be passed through a significant amount of ion-exchange resin, usually in the form of a filter bed, making it effective, in most cases, for only treating small volumes of waste water. The complex fabrication process and sophisticated synthetic chemistry required by ion exchange metal recovery technology significantly contributes to the expense of its use to purify liquid waste streams. The cost and complexity of ion exchange also limits the variety of resins available.
Although ion exchange resin beds may be regenerated, the waste waters from regeneration must often be retreated to remove bulk contaminants and then usually passed through the ion exchange resin again to eliminate hazardous materials. Thus, ion exchange is a cumbersome process, and therefore impractical, especially for large volumes of waste water in a continuous-treatment process, as compared to using ion-exchange in a batch-treatment process.
Electrolysis is also expensive, requires significant maintenance, employs other resources, may create its own waste disposal problems and is energy intensive. Electrolytic recovery is, at best, 70%-80% efficient. Besides, the electrolyte systems available today are very sensitive to the presence of contaminants.
Use of either ion exchange or electrolytic recovery of metals from waste streams requires separation of streams for processing, thereby ultimately creating multiple waste streams. This multiplicity of streams results in a costly waste removal process for the waste stream generator.
In contrast to the ion exchange and electrolytic metal recovery processes, one of the more acceptable technologies for treating waste water is based on a settling process, using fixating agents such as hydroxides and sulfates. The fixating chemicals are added to water in a settling tank to absorb or otherwise transform the contaminants into materials which settle to the bottom of the tank. This technology uses comparatively simple equipment and permits the processing of large volumes of waste waters, without adding materials which would result in an environmentally undesirable effluent stream. However, in many cases, use of ordinary settling processes fails to reduce contaminant concentrations to levels low enough to meet the statutory requirements, without using excessive amounts of materials, over a protracted processing time. Current settling processes often produce undesirably large quantities of solid hazardous or toxic wastes in the form of sludge. The sludge cannot, for the most part, be effectively regenerated. Thus, using current settling techniques for waste water treatment, the resulting sludge product is yet another waste material that must be disposed of in a secure landfill without benefit of recycling. In turn, this process results ultimately in the necessity to clean the environment in the long term future.
As a result of problems associated with the above noted technologies, waste water generators have been forced to consider alternative methods which employ the addition of metal complexing agents to waste water streams and sludge of various industrial processes.
For example, U.S. Pat. No. 3,966,601 (Stevenson, et al.) discloses a purification process comprised of mixing a soluble heavy metal salt and a heavy metal dithiocarbamate. U.S. Pat. No. 4,387,034 (Unger, et al.) discloses a collector for use in concentrating metal values in ores by flotation, the collector being comprised of a mixture of 0-isopropyl N-ethylthionocarbamate and o-isobutyl N-methylthionocarbamate.
U.S. Pat. No. 4,578,195 (Moore, et al.) discloses a process for treating aqueous effluents to remove polluting metallic elements wherein the effluent is contacted with a poly(dithiocarbamate) chelating agent. U.S. Pat. No. 4,612,125 (Elfline) discloses a method for removing heavy metals from waste water streams, comprising treating the waste water with sulfur-containing compounds, such as sodium tri-thiocarbamate.
U.S. Pat. No. 4,678,584 (Elfline) discloses a method for treating a liquid containing a heavy metal comprising contacting the liquid with a mixture of sodium diethyldithiocarbamate and sodium tri-thiocarbanate. U.S. Pat. No. 4,943,377 (Legare) discloses a method for removing heavy metals from waste effluents comprising mixing the effluents with a solution of a sulfur compound such as sodium polythiocarbamate. U.S. Pat. No. 5,372,726 (Straten) discloses a method for treating water polluted by metal ions comprising the steps of adding thiocarbamide, potassium or sodium hydroxide, and potassium or sodium hyposulfite.
U.S. Pat. No. 5,264,135 (Mohn) discloses a method for treating sludge from industrial waste water streams comprising the steps of adding a metal complexing agent to the sludge such as dimethyl-dithiocarbamate or a salt thereof. The metal complexing agent is added to a sludge thickening tank prior to de-watering in a filter press to form a sludge that contains 60% to 85% moisture by weight. Mohn does not disclose use source separation of the effluent throughout the process and does not disclose adjusting the pH of the waste solution to the optimal point of insolubility for the various metals involved. Mohn characterizes the sludge as being fixated, thereby allowing disposal in landfills.
In addition, a number of metallurgical processes for recovering metal have also been disclosed. For example, U.S. Pat. No. 3,899,322 (Yosim et al.) discloses a process for recovery of noble metals from scrap comprising melting the scrap at a temperature between 800.degree. F. and 1,800.degree. F. U.S. Pat. No. 4,135,923 (Day) discloses a process for the extraction of metals from metallic materials comprising heating a lead-free mixture of metals and separating the metals in a molten state.
U.S. Pat. No. 5,008,017 (Kiehl, et al.) discloses a process for recovering metals from waste liquids, including a step for obtaining pure metal. A dewatered sludge is heated for a period from about thirty minutes to about one hour at 900.degree. F., to recover substantially pure silver. However, this metallurgical process for recovering metals from a metallic sludge is very complicated, and requires a metal complexing agent be applied to the metallic sludge of waste streams.
None of the known prior art technologies separate and also recover a variety of metals from one or more waste streams in order to use the metals as valuable commercial products, nor do they disclose the recovery, recycling, and reuse of the recovered metals. In those prior art processes using reagents to cause fixation of metals and to produce a fixated hydroxide sludge byproduct, the resulting byproducts must be sent to and disposed of in a secure landfill or alternative receiving site.
For the foregoing reasons, there is a need for a method for removing, separating, and recovering metals and groups of metals, such as transition metals, alkali metals, and alkaline earth metals. An efficient method for removing, separating, and recovering such metals in a cost effective manner with a high degree of recovery from waste streams and with minimal amounts of unprocessed solids and sludge remaining in the environment is needed. Illustrative, but not limitative, of the metals that such a method can be capable of separating, removing and recovering are such precious and non-precious metals as aluminum, barium, beryllium, calcium, chromium, cobalt, copper, gold, iron, lead, magnesium, manganese, nickel, platinum, silver, tin, vanadium, zinc, and the like.
Such a process should also be capable of removing other metals such as antimony, arsenic, selenium, thallium, and the like from waste streams with at least 50% removal.